1. Field of the Invention
The present invention relates to a pressure sensitive adhesive for sticking an electromagnetic wave-shielding film and an optically functional film, and a display panel filter element containing the same. The pressure sensitive adhesive according to the present invention can be used for the manufacture of a display panel filter element constituting, for example, a plasma display panel or the like.
2. Description of the Related Art
It is believed that an electromagnetic wave emitted from an electromagnetic device such as a plasma display affects not only other electromagnetic devices, but also the human body. Particularly, an electromagnetic wave having a frequency of 30 MHz to 130 MHz is emitted from a plasma display, and may affect computers and computer peripherals positioned therearound. Accordingly, the emitted electromagnetic wave should be prevented from leaking toward the outside. An electromagnetic wave may be shielded, for example, by a method of covering an emitting source with a case formed from a highly conductive material, or a method of applying a conductive net on the emitting source. However, an apparatus such as a plasma display, which must be observed, requires transmittability, and the above methods cannot be adopted.
Therefore, in a plasma display or the like, an electromagnetic wave-shielding sheet composed of a transparent substrate film and a metal foil mesh carried thereon via an adhesive layer is used as a means having not only transmittability but also a property of shielding an electromagnetic wave. The metal foil mesh is formed by mounting a metal foil on the transparent substrate film via the adhesive layer, and forming pores by etching the metal foil. The electromagnetic wave-shielding film, which is a laminate of the transparent substrate film, the adhesive layer, and the metal foil mesh, can provide a required shielding property by appropriately adjusting a thickness of the metal foil or a size of the mesh pores, even if the strength of an emitted electromagnetic wave is high as in a plasma display. Further, it also has a sufficient transparency to assure a visibility of the display screen (see, for example Patent Document No. 1).
The electromagnetic wave-shielding film is stuck together with various optically functional films, such as a near infrared light-absorbing film, an ultraviolet light-absorbing film, or an antireflective film by a pressure sensitive adhesive applied onto the metal foil mesh on the surface of the electromagnetic wave-shielding film. After sticking together the electromagnetic wave-shielding film and the optically functional film, an autoclaving treatment is generally carried out under a pressure of about 0.3 to 1.5 MPa at about 40 to 80° C. In the autoclaving treatment as above, the inside of the pores of the metal foil mesh having a fine concave-convex surface structure is entirely filled with the pressure sensitive adhesive under pressure, to entirely exclude bubbles; this property will be hereinafter sometimes referred to as the “filling-in property”. Further, the pressure sensitive adhesive used in the above sticking must have a sufficient adhesive strength between the metal foil mesh on the surface of the electromagnetic wave-shielding film and the optically functional film, and a sufficient adhesive strength between the optically functional film and the adhesive layer (for the metal foil) exposed in the pores of the metal foil mesh so that a separation of the electromagnetic wave-shielding film from the optically functional film is prevented.    [Patent Document No. 1] Japanese Unexamined Patent Publication No. 2003-188576.